Various parts of the Western Cape were recently subjected to raging wild fires, threatening farmland, vineyards and houses aggravated by strong south-easterly winds and very hot temperatures. The damage seems to be mostly limited to fynbos and other vegetation, but the influence of the smoke on ripening grapes presents a much more complex predicament. ELONA HESSELING explains.
Some of the areas affected by these recent fires, with their thick clouds of smoke and ash, were Clanwilliam and the Cederberg Mountains, Franschhoek and Paarl along the Wemmershoek Mountains, Overstrand, Tulbagh and the Banghoek Valley near Stellenbosch.
But, this is nothing new and these veld fires are evidently more frequently causing devastation in South Africa, as well as Australia, Canada and the USA. In 2009, many South African wine farms sustained a loss of vineyards and grapes, either directly due to the fires or as a result of smoke taint.
The degree of smoke taint is largely determined by smoke density, duration of smoke exposure and the timing of exposure, influencing the ultimate wine quality. Substantial research has been done lately on this subject, especially in Australia, but a certain element of unpredictability remains when it comes to the specific impact of smoke on wine quality.
Vergelegen winemaker André van Rensburg had to face massive loss and damage following the 2009 fires in the Helderberg area. He explains that the type of vegetation that burns plays a significant role in the degree of smoke taint.
“Fynbos burns quickly, producing less smoke so even when it burns close to a vineyard, you hardly get any smoke taint. But alien vegetation, such as pines and eucalyptus, produces massive amounts of smoke and deposits its oils on the vineyards – when these burn, you almost have a 100% chance of smoke taint problems,” he says.
The compounds most commonly believed to cause smoke taint are guaiacol and 4-methylguaiacol. However, exposed grapes can also have elevated levels of compounds such as 4-ethylguaiacol, 4-ethylphenol, eugenol and furfural, as well as other volatile phenols.
The distance of the fire from the vineyard also plays an important role. The Helshoogte-area of Stellenbosch was affected by wild fires in 2009 and Tokara winery sustained the loss of vineyards, as well as grapes and wine due to smoke taint.
Winemaker Miles Mossop explains that the fire was burning very close to their Pinotage block, which suffered the most from smoke taint. “The effect on the Cabernet Sauvignon and Shiraz blocks right next to it was much less and we experienced no problems in the resultant wines,” he says.
Another vital factor that influences the degree of smoke taint is the length of time that the vineyards and grapes are exposed to the smoke. According to an article Smoke Taint in Grapes and Wine, published in the February 2010 Wine Business Monthly, research shows that a single heavy exposure of smoke to grapevines for 30 minutes is enough to result in wine with smoke taint. However, repeated smoke exposure or smoke exposure over a long period of time has a cumulative effect.
Australian smoke taint expert and director of Memstar, David Wollan, explained in correspondence with Wine- Land that the timing of smoke exposure is very important. He found that although grapes are vulnerable at any time after flowering, the period of the greatest susceptibility is just after veraison until the end of harvest.
At a Smoke Taint Symposium held in Melbourne, Australia, last year Dr Nicole Cain’s presentation (available on www.awri.au) showed that although the leaves take up compounds in trace amounts, the process takes place primarily through the berries. This means that the uptake can also be slightly reduced in a vineyard with a denser canopy.
According to research by Kristen Kennison of the Department of Agriculture and Food for Western Australia, smoke compounds are found primarily within the skins of grape berries and not within the berry pulp itself.
David explains that most of the taint in grapes is in a bound form, predominantly as non-volatile glycosides, which cannot be tasted in grapes – trial fermentation of crushed grape samples are necessary to test for smoke taint. “It is only after fermentation that these break down and the offensive flavors become apparent.”
After the 2009 fires, André took a similar approach to assess smoke damage. They looked for traces of ash or smoky, burnt smells in the canopies and these vines were then marked. They sampled berries, did trial fermentation and fining, but to no avail. “Nothing helped and in the end we had to dump about 20% of our total harvest – the grapes were either 100% fine or totally screwed up,” he says.
A study commissioned by the Australian Wine Research Institute (AWRI) recently found a new successful way of assessing the extent of smoke taint in grapes and wine. This diagnostic assay relies on measuring the quantity of glycosidic grape metabolites, which are created by the major volatile phenols in smoke. This is more suitable to identify smoke exposure and assess its impact than the existing guaiacol and 4-methylguaiacol levels.
Although wild fires cannot be controlled, Dr Renata Ristic believes that the intensity of smoke taint can, to a certain extent, be reduced by means of vineyard management. She presented her study results at the Smoke Taint Symposium and found that while defoliation prior to smoke exposure led to intense smoky characters, it resulted in a reduced intensity of smoky flavors and an enhanced perception of fruit when done after smoke exposure.
In an article by Charl Theron, published in the May 2009 Wynboer, he explained that the smoky compounds in wine will increase with increased skin contact. Because of this, white grapes are usually less of a risk, as long as they are handled with care, receive little or no skin contact and are whole-bunch pressed.
Even though red grapes are usually more problematic, Miles decided to make wine with their smoke-affected Mourvdre and Pinotage grapes. “It had high levels of guaiacol, but we used only the free run and lightly pressed juice,” he says.
The Pinotage went into new French oak barrels and after 16 to 18 months, the burnt flavours intensified, due to the wood that also releases guaicol and 4-methylguaiacol into wine. “The Pinotage was quite heavily affected, but the flavours were slightly masked by the oaky character. Mourvdre is a less intense wine and the burnt flavours were very obvious in it,” he says.
These ‘burnt’ flavours often associated with smoke taint can include anything from smoky, burnt, charred and ashtray characters, to bacon, smoked sausages, coffee and burnt rubber. According to the Wine Business Monthly article, the concentration levels and perception of guaiacol and 4-methylguaiacol can increase during wine processing and ageing. It was noted that these levels can increase up to three times during fermentation, while it also keeps regenerating during ageing.
Various treatments have been put to the test with affected wines, but most of them with little or no success. However, according to the Wine Business Monthly article, some practical methods, such as minimal extraction, processing affected grapes separately, hand harvesting and minimal skin contact helped to manage and reduce smoke taint to a degree.
One of the best solutions to date seems to be a membrane system developed by Memstar, which noticeably lowers the smoke taint compounds in wine by means of reverse osmosis. Columbit is the local supplier and Miles used the system to remove as much of the taint as possible, without stripping too much flavour from the wine.
With this method, smoke taint compounds can be reduced to below the sensory threshold, but not completely removed without negatively impacting the wine’s quality it is also an expensive treatment, so a decision needs to be made on where and when to cut your losses. Although it managed to remove many of the compounds, the quality was still a little below average and the Pinotage had to be sold in bulk.
David explains that, with Memstar, they have treated many millions of litres of wine for clients since 2003 “the recovered wines were all commercially acceptable, compared to what would have been a complete loss. Recently I retasted some production samples from the 2009 vintage and the post treatment wines were clearly superior to the untreated wines,” he says. “Contrary to popular belief, the taint had not returned.”
However, at the moment, there seems to be no complete and magical method of ridding wines of smoke taint without compromising the quality. Andr believes that the best we can do is to try and play a preventative role. “Farmers must keep their farms clean from alien vegetation, while maintaining fire paths and equipment,” he says. “You should always be ready have a team on call and educate workers on how to fight fires, as well as how to prevent accidental fires.”
No matter how advanced our knowledge, research and understanding of the impact of smoke taint on wine quality has become, it remains an unpredictable and sometimes irrevocable problem.
The extent of damage that the recent fires caused is still relatively unknown, although it seems that most farms were lucky enough to escape serious and permanent damage. However, the impact that the thick, orange cloud of smoke which encapsulated certain areas for days had on the grapes could still be cause for numerous headaches and losses.